arXiv subjects

Dejan Stojkovic

Publications and source records attributed to Dejan Stojkovic.

At least 19 recordsLinked to original sources

arXiv2026

A Multi-Axion Ladder Across Cosmic History: From Inflation, BBN, and Early Dark Energy to Late-Time Accelerated Expansion

What if dark energy is recurrent throughout cosmic history? In this picture, episodes of scalar-field dark energy become less surprising and more natural features of cosmic evolution. We construct a homogeneous multi-axion cosmology with a transient contribution during Big Bang nucleosynthesis, two early dark energy components before recombination, and a thawing field that supplies the present dark-energy density. As Hubble friction weakens, the fields begin to roll at successive epochs set by their potential-curvature scales. Their initial displacements affect the rolling delays and peak energy fractions. The transient fields have third-power cosine potentials, whose sextic minima allow faster-than-radiation dilution during rapid, small-amplitude oscillations. All four fields are evolved in a common Friedmann background, with their first roll and subsequent dynamics resolved numerically. With reference matter and radiation densities taken from Planck 2018, the benchmark has a nucleosynthesis-era peak fraction of approximately $0.99\%$ near $z=10^9$. The two early dark energy fields peak near $z=7.9\times10^3$ and $2.4\times10^3$, with individual fractions of $8.6\%$ and $7.8\%$; their combined fraction reaches $9.7\%$. The late-time field is normalized to supply a present fraction of approximately $0.685$, while the three transients leave a combined fraction of approximately $6.0\times10^{-7}$. An aligned two-axion example illustrates the enhanced field range available for an inflationary extension. We also outline how searches for transient contributions to the expansion rate at other epochs could constrain additional axion scales.

astro-ph.CO↗

arXiv2026

Information capacity of quantum statistics: Fock-state tests of a discrete binary-sequence model on cloud photonic quantum processors

Our central premise is that quantum mechanics may be the statistical limit of a more fundamental discrete theory: any such theory equips a physical system with a finite information capacity, and its departure from quantum statistics is controlled by how much of that capacity the system uses. We show that commercial cloud photonic quantum processors have reached the precision required to bound this capacity from below, using the binary-sequence model of Powers et al. as the concrete test theory: outcome probabilities arise from counting discrete sequences of length $n$, quantum mechanics is recovered as $n \to \infty$, and $n$ measures the information capacity of the register behind a prepared state. Photon Fock states $|1\rangle$, $|1,1\rangle$, heralded $|2\rangle$, and cascaded beam-splitter pairs are measured on programmable interferometers with dominant systematics determined in situ. The model's composition-consistent parametrization, singled out by requiring that rotations compose, recovers quantum mechanics with deviations $1.24/n$; a random-effects likelihood analysis calibrated by parametric bootstrap excludes all $n \le 100$: the information capacity of the register carrying the two-photon state, if finite, exceeds $10^2$. Cascaded beam splitters test the composition law directly: the data are split-invariant, excluding naive count composition at $8\sigma$ and confirming the interference-sign rule. Model-independently, curve-averaged deviations from the quantum partition law larger than $2.3\times10^{-2}$ are excluded at 95% CL, and the originally published linear parametrization is excluded outright. Because the compilation offset is frozen per circuit it is calibratable, opening the $10^{-3}$ floor ($n \sim 10^3$) to current hardware: cloud photonic processors are quantitative instruments for quantum foundations, and information capacity an experimentally boundable quantity.

quant-ph↗

arXiv2026

The quantum harmonic oscillator from binary sequences

The quantum harmonic oscillator (QHO) is normally built on a structure that contains a Hilbert space, ladder operators, and a Born rule. Here we derive it from counting. We adopt three information-theoretic postulates: information is carried by binary sequences of length $n$; only symbol counts, not the sequences themselves, are observable; and the transition measure must account for exactly the $2^n$-configuration capacity of the register. The first two postulates force the counting problem into the Hamming/Delsarte association scheme and endow it with an exact symmetry under relabeling of the two symbols. Given an exchange-symmetric bilinear transition measure we prove a rigidity theorem: relabeling symmetry and normalization together force the alternating sign $(-1)^t$ which drives interference. The variable that carries the sign, i.e. the overlap between input and output sequences, or equivalently their Hamming distance $d_{ab}$, is a hidden quantum number of the transition as a whole, defined only by the two endpoints jointly and unknowable from either alone. Interference thus enters as alternating-sign bookkeeping over a nonlocal, relational variable $d_{ab}$, rather than as a separate dynamical ingredient. This forced weighting makes the transition probability proportional to the square of a Krawtchouk polynomial, whose difference equation has an exactly equally spaced spectrum and which converges to the QHO as $n\to\infty$. At fixed excitation, deviations from the QHO are $\mathcal{O}(1/n)$, and are testable in laboratory realizations of the oscillator, since any real system has finite information capacity. In this framework, interference, quadratic (Born-rule-like) probabilities, and the particle-hole symmetric oscillator spectrum are consequences of counting; only the bilinear form of the measure is assumed (though well motivated) rather than derived.

physics.gen-ph↗

arXiv2026

Emergent Space, Time, and Lorentz Symmetry from Binary Sequences

We construct an information theory framework in which the fundamental objects are binary sequences of length $n$, equipped with the bitwise XOR operation. The only physical observables are counts of XOR-generated symbol classes, while the exact locations of symbols are inaccessible. Averaging over those locations ultimately yields the Minkowski interval as an invariant object that maximizes (information) entropy. Correlations between two binary sequences are base-4 sequences that we label as ``events'', and events are connected with maps. The entire kinematic structure of Special Relativity is recovered under minimal assumptions, i.e. counts that represent space and time increments carry equal informational weight. The central claim is that Lorentz symmetry is the typical large-n behaviour of XOR counting. At finite $n$ the framework yields a discrete rapidity spectrum, a bound $\gamma_{\max} = O(\sqrt{n})$, and interval fluctuations of relative size $O(n^{-1/2})$, with standard special relativity recovered as $n \to \infty$. However, the light cone and one null coordinate are exact for every microscopic configuration, so the symmetry group is undeformed and dispersion relations are unmodified. Thus, the theory, though discrete, implies no Lorentz violation of the standard phenomenological kind. Ultra-high-energy cosmic rays already require $n \gtrsim 10^{23}$; interferometry excludes the variant in which $n$ scales linearly with system size, leaving a holographic area law. The most striking prediction of the framework concerns systems at the maximum of their information capacity, where $n$ is necessarily finite: black holes and de Sitter space. There the corrections are of order one within a Planck proper length of the horizon, regardless of the horizon's size, and the spacetime description fails altogether at the endpoint of black-hole evaporation, where $n$ itself is of order unity.

physics.gen-ph↗

arXiv2026

Signals of Doomsday III: Cosmological signatures of the late time $U(1)_{EM}$ symmetry breaking

Of the universe's original gauge symmetries, only $SU(3)_c$ (quantum chromodynamics) and $U(1)_{\rm EM}$ (electromagnetism) remain unbroken today. There is, however, no reason to assume that these symmetries are permanent. This paper explores the potential astrophysical observational signatures of a late-time breaking of $U(1)_{\rm EM}$. We present a model with a new massive scalar field whose potential supports a first-order phase transition through the nucleation of true-vacuum bubbles. If the propagation of the bubble walls slows down due to interactions with the surrounding matter and radiation, these signals can reach us before the bubble wall itself arrives. Using the vacuum-mismatch method, we calculate the spectrum of particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at terminal wall velocity generates a large population of thermally produced scalars and massive photons, which continues even after the mismatch channel shuts off. We then use event generators to simulate the decays of the new scalar and the massive photon into Standard Model particles and obtain, as the final result, the energy spectra of photons and neutrinos. Since the dominant final decay products after hadronization and the decay of unstable particles are photons and neutrinos, they act as long-range signatures of the transition. We also estimate the possible lead time of these photon and neutrino signals relative to the arrival of the bubble wall itself, showing that even a modest subluminal wall velocity can in principle provide an observable precursor. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high-energy photons and neutrinos, which could in principle be detectable from sufficiently nearby bubbles with present or future multi-messenger facilities.

hep-ph↗

arXiv2026

Modified Teukolsky formalism: Null testing and numerical benchmarking

Next-generation gravitational-wave detectors will make black-hole ringdown an increasingly sensitive probe of small departures from General Relativity in the strong-field regime. This motivates obtaining high-precision predictions of gravitational effective field theory, as spectral shifts can be quite small. Here we perform a focused stress test of the modified-Teukolsky framework by designing two null diagnostics. First, we consider an action with redundant operators that must produce zero first-order vacuum QNM shifts. Second, we exploit a Ricci-flat identity relating two physical cubic Riemann to test such a relation is satisfied by the ringdown spectra obtained. We compute the shifts using two independent numerical approaches: the eigenvalue-perturbation and generalized continued-fraction (Leaver-type) methods. Both null tests are passed across multiple multipoles and overtones, and the control-operator results agree in magnitude with the benchmark values reported in Ref. [1]. These validations support using the framework for obtaining accurate predictions for robust strong-field tests, with straightforward extensions to rotating backgrounds and coupling with matter fields.

gr-qc↗

arXiv2025

Coupling between gravitational and electromagnetic perturbations on Kerr Spacetime

We extend our previous Schwarzschild metric-based studies of gravitational--electromagnetic (GEM) coupling to rotating black holes by working directly in a curvature-based Newman--Penrose/Teukolsky framework on Kerr spacetime. Within a minimally coupled Einstein--Maxwell system, we derive explicit quadratic electromagnetic source terms for the spin-$-2$ Teukolsky equation, providing a foundation for future numerical studies of GEM interactions in the framework of black-hole spectroscopy. Moreover, we give order-of-magnitude arguments showing that GEM quadratic quasinormal modes (QQNMs) can become relevant in a range of charged and magnetized astrophysical scenarios. Finally, we show through a brief dilaton-theory example that the GEM QQNM spectrum is sensitive to how gravity couples to electromagnetism, thereby providing a model-based way to test minimal coupling and to constrain hidden $U(1)$ sectors with gravitational-wave observations.

gr-qc↗

arXiv2025

The signals of doomsday II: Cosmological signatures of late time $SU(3)_c$ symmetry breaking

Only two gauge symmetries remain unbroken today: $SU(3)_c$ and $U(1)_{\rm EM}$. Both are crucial to our universe as it is and to our form of life. Unless we are special observers at the end of the cosmological symmetry-breaking chain, there is no reason they must remain unbroken forever. We investigate the cosmological signatures of late-time $SU(3)_c$ breaking in our model with a new colored scalar whose potential supports a first-order phase transition through true-vacuum bubble nucleation. We analyze three physically distinct production mechanisms of the expanding bubble wall. The first is direct nonthermal production from vacuum mismatch across the accelerating wall in the scalar and massive-gluon sectors. The second arises from frictional dissipation into a shocked layer. Including the finite-temperature effective potential, we show that our benchmark critical temperature $T_c$ is below the massive-gluon and scalar masses; an equilibrated relativistic broken-phase thermal bath would therefore restore $SU(3)_c$. We formulate a subcritical thermal freeze-out estimate below $T_c$, where massive broken-phase particles can still be produced but only with Boltzmann suppression. The third is a non-equilibrium wall--matter transition-radiation channel: ambient baryonic matter crossing the relativistic wall can radiate broken-phase massive gluons and scalar excitations. This requires no thermal bath above $T_c$ and can also continue after the wall reaches terminal velocity. We study the decays of the physical color-octet scalar and massive gluons, using \texttt{Pythia} to hadronize their decay products and determine the resulting photon and neutrino spectra. If the wall reaches a subluminal terminal velocity, these particles can arrive before the wall. The resulting high-energy spectra are a long-range observational signature that, if observed, could signal cosmic doomsday.

hep-ph↗

arXiv2025

Quantum black holes: inside and outside

For a unitary description of an evaporating black hole, one usually chooses the time slices that cover only outside of the event horizon, which is mostly problem-free because the event horizon is not encountered. However, is there any justification for avoiding time slices that cover inside the event horizon? To answer the question, we investigate the Wheeler-DeWitt equation, where the time slices can cover both inside and outside the event horizon. We find that one can reasonably construct a wave packet that covers outside, but the wave function must be annihilated near the event horizon. This observation strongly suggests that we cannot choose a coherent state for a spacelike hypersurface that crosses the event horizon. To explain the unitary time evolution, we must keep the slices as coherent states; hence, they must always be outside the event horizon. In contrast, inside the horizon, we cannot have a single coherent state of a classical spacetime. Hence, the interior must be a superposition of several coherent states, which implies that there exists a horizon-scale uncertainty and a black hole should be viewed as a highly quantum macroscopic object. We provide a synthetic approach to understanding the information loss paradox from this perspective.

gr-qc↗

arXiv2025

The Signals of Doomsday I: False Higgs vacuum decay signatures

The measured standard model parameters indicate that we might live in a false Higgs vacuum, though with a very long lifetime. However, small black holes can serve as catalysers and significantly speed up the phase transition. In fact, bubbles of true vacuum might already exist in our universe. If the propagation of the bubble walls slows down due to interaction with the surrounding matter and plasma, these signals can reach us before the bubble wall hits us. Using the vacuum mismatch method, we calculate the spectrum of the Higgs particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at the terminal wall velocity generates a large population of thermally produced Higgs particles, which continues even after the mismatch channel shuts off. Since the Higgs is neutral, a good part of the final decay products (after hadronization, annihilation and decay of unstable particles) will be photons and neutrinos, which will then act as a long-range signature. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high energy neutrinos and photons from Higgs decays, which could be detectable from sufficiently nearby bubbles with current or upcoming multi messenger facilities.

hep-ph↗

arXiv2024

More Nonlinearities? II. A Short Guide of First- and Second-Order Electromagnetic Perturbations in the Schwarzschild Background

We study second-order electromagnetic perturbations in the Schwarzschild background and derive the effective source terms for Regge-Wheeler equation which are quadratic in first-order gravitational and electromagnetic perturbations. In addition to the induced mixed quadratic modes, we find that linear gravitational modes are also excited, with amplitudes dependent on the electromagnetic potential. A toy model involving a Dirac delta function potential demonstrates mixing of linear gravitational and electromagnetic perturbations with frequencies $ \omega^{(1)} $ and $ \Omega^{(1)} $, resulting in the second-order QNM mixing in the electromagnetic field at $ \Omega^{(2)} =\Omega^{(1)} + \omega^{(1)} $. This complements prior work in \cite{aly2024nonlinearities} on the second-order gravitational perturbation mixing and highlights potential applications in multi-messenger astrophysics for systems observed by LIGO-Virgo-KAGRA (LVK) and upcoming LISA. We also study first-order perturbations due to a point charge and show it could be reduced to a one-dimensional path integral. Within the toy model, we investigate the first-order electromagnetic perturbation due to a radially free-falling single charge $ q $ and radial dipole moment $ p = q \eta $, employing semi-analytical and numerical methods. For the dipole case, we show that the QNM perturbation is excited with a nearly constant amplitude. Future work will focus on incorporating mixing in more realistic potentials and exploring numerical approach in the context of rotating spacetimes.

gr-qc↗

arXiv2024

More Nonlinearities? Electromagnetic and Gravitational Mode Mixing in NSBH Mergers

We investigate the possibility of electromagnetic fields leaving imprints on gravitational wave (GW) signals from Neutron Star-Black hole (NSBH) mergers, specifically in the context of extreme mass ratio inspirals (EMRIs). Using black hole perturbation theory (BHPT) in the context of a minimally coupled Einstein-Maxwell system, we demonstrate that electromagnetic quasi normal modes(QNMs) can excite gravitational QNMs with frequencies that are linear or quadratic in the electromagnetic QNMs, at first level of mixing. Moreover, We then study the electromagnetism-gravity coupling by approximating the Regge-Wheeler and Zerilli potentials with Dirac delta functions. In this example, we examine gravitational perturbations induced by the electromagnetic field of an ideal dipole radially free fall towards the blackhole, building on calculations from a companion paper [1]. Our results show that both linear and quadratic electromagnetic QNMs appear in gravitational perturbations. In addition, linear gravitational QNMs are also excited due to the electromagnetic source, with their amplitudes depending on the details of the electromagnetic and gravitational potentials, analogous to gravitational mode mixing analysis. Furthermore, at late stages, gravitational perturbations might exhibit polynomial tails induced by electromagnetic perturbations. This article sets the stage for future numerical investigations aimed at identifying such modes in various scenarios.

gr-qc↗

arXiv2024

Searching for small primordial black holes in planets, asteroids and here on Earth

Small primordial black holes could be captured by rocky planets or asteroids, consume their liquid cores from inside and leave hollow structures. We calculate the surface density and surface tension of a hollow structure around a black hole and compare them with the density and compressive strength of various materials that appear in nature to find the allowed parameter space. For example, granite or iron can support a hollow asteroid/planetoid/moon of the size of up to $0.1 R_\oplus$. Along the same lines, future civilizations might build spherical structures around black holes to harvest their energy. Using the strongest material that we currently know how to make (multiwall carbon nanotube), to withstand gravity of one solar mass black hole, the shell must be constructed at distances larger than $10^4 R_\odot$. Alternatively, a fast black hole can leave a narrow tunnel in a solid object while passing through it. For example, a $10^{22}$g black hole should leave a tunnel with a radius of $0.1$ micron, which is large enough to be seen by an optical microscope. We could look for such micro-tunnels here on Earth in very old rocks, or even glass or other solid structures in very old buildings. While our estimate gives a very small probability of finding such tunnels, looking for them does not require expensive equipment and long preparation, and the payoff might be significant.

gr-qc↗

arXiv2024

Does DESI 2024 Confirm $\Lambda$CDM?

We demonstrate that a $\sim 2 \sigma$ discrepancy with the Planck-$\Lambda$CDM cosmology in DESI Luminous Red Galaxy (LRG) data in the redshift range $0.4 < z < 0.6$ with effective redshift $z_{\textrm{eff}} = 0.51$ translates into an unexpectedly large $\Omega_m$ value, $\Omega_m = 0.67^{+0.18}_{-0.17}$. We independently confirm that this anomaly drives the preference for $w_0 > -1$ in DESI data \textit{alone} confronted to the $w_0 w_a$CDM model. Given that LRG data at $z_{\textrm{eff}} = 0.51$ is at odds with Type Ia supernovae in overlapping redshifts, we expect that this anomaly will decrease in statistical significance with future DESI data releases leaving an increasing $\Omega_m$ trend with effective redshift at higher redshifts. We estimate the current significance of the latter in DESI data at $\sim 1.8 \sigma$ and comment on how it dovetails with independent observations. It is imperative to understand what makes DESI LRG data at $z_{\textrm{eff}} = 0.51$ an outlier when it comes to $\Omega_m$ determinations.

astro-ph.CO↗

arXiv2024

Putting Flat $Λ$CDM In The (Redshift) Bin

Flat $Λ$CDM cosmology is specified by two constant fitting parameters at the background level in the late Universe, the Hubble constant $H_0$ and matter density (today) $Ω_m$. Mathematically, $H_0$ and $Ω_m$ are either integration constants arising from solving ordinary differential equations or are directly related to integration constants. Seen in this context, if fits of the $Λ$CDM model to cosmological probes at different redshifts lead to different $(H_0, Ω_m)$ parameters, this is a mismatch between mathematics and observation. Here, in mock observational Hubble data (OHD) (geometric probes of expansion history) we demonstrate evolution in distributions of best fit parameters with effective redshift. As a result, considerably different $(H_0, Ω_m)$ best fits from Planck-$Λ$CDM cannot be precluded in high redshift bins. We explore if OHD, Type Ia supernovae and standardisable quasar samples exhibit redshift evolution of best fit $Λ$CDM parameters. In all samples, we confirm a decreasing $H_0$ and increasing $Ω_m$ trend with increasing bin redshift. Through comparison with mocks, we confirm that similar behaviour can arise randomly within the flat $Λ$CDM model with probabilities as low as $p = 0.0021$ ($3.1 \, σ$). We present complementary profile distribution analysis confirming the shifts in cosmological parameters in high redshift bins. In particular, we identify a redshift range where Planck $(H_0, Ω_m)$ values are disfavoured at $99.6 \%$ ($2.9 σ$) confidence level in a combination of OHD and supernovae data.

astro-ph.CO↗

arXiv2024

Superluminal propagation along the brane in space with extra dimensions

We demonstrate that a model with extra dimensions formulated in Csaki et al. (Phys Rev D 62, 045015), which fatefully reproduces Friedmann-Robertson-Walker (FRW) equations on the brane, allows for an apparent superluminal propagation of massless signals. Namely, a massive brane curves the spacetime and affects the trajectory of a signal in a way that allows a signal sent from the brane through the bulk to arrive (upon returning) to a distant point on the brane faster than the light can propagate along the brane. In particular, the signal sent along the brane suffers a greater gravitational time delay than the bulk signal due to the presence of matter on the brane. While the bulk signal never moves with the speed greater than the speed of light in its own locality, this effect still enables one to send signals faster than light from the brane observer's perspective. For example, this effect might be used to resolve the cosmological horizon problem. In addition, one of the striking observational signatures would be arrival of the same gravitational wave signal at two different times, where the first signals arrives before its electromagnetic counterpart. We used GW170104 gravitational wave event to impose a strong limit on the model with extra dimensions in question.

gr-qc↗

arXiv2023

On the generalization of the Kruskal-Szekeres coordinates: a global conformal charting of the Reissner-Nordstrom spacetime

The Kruskal-Szekeres coordinates construction for the Schwarzschild spacetime could be viewed geometrically as a squeezing of the $t$-line associated with the asymptotic observer into a single point, at the event horizon $r=2M$. Starting from this point, we extend the Kruskal charting to spacetimes with two horizons, in particular the Reissner-Nordström manifold, $\mathcal{M}_{RN}$. We develop a new method for constructing Kruskal-like coordinates and find two algebraically distinct classes charting $\mathcal{M}_{RN}$. We pedagogically illustrate our method by constructing two compact, conformal, and global coordinate systems labeled $\mathcal{GK_{I}}$ and $\mathcal{GK_{II}}$ for each class respectively. In both coordinates, the metric differentiability can be promoted to $C^\infty$. The conformal metric factor can be explicitly written in terms of the original $t$ and $r$ coordinates for both charts.

gr-qc↗

arXiv2023

Spacetime surgery for black hole fireworks

We construct an explicit model for the black hole to white hole transition (known as the black hole fireworks scenario) using the cut-and-paste technique. We model a black hole collapse using the evolution of a time-like shell in the background of the loop quantum gravity inspired metric. We then use the space-like shell analysis to construct the firework geometry. Our simple and well defined analysis removes some subtle issues that were present in the previous literature. In particular, we demonstrate that the null energy condition must be violated for the bounce. We also calculate the proper time scales required for the black to white hole transition, which in any valid scenario must be shorter than the evaporation time scale. In contrast, we show that the bouncing time for the distant observer can be chosen arbitrarily, since it is determined by how one cuts and pastes the spacetimes outside the event horizon, and thus does not have any obvious connection to quantum gravity effects.

gr-qc↗